“Through‐Space” Relativistic Effects on NMR Chemical Shifts of Pyridinium Halide Ionic Liquids

Abstract

We have investigated, using two‐component relativistic density functional theory (DFT) at ZORA‐SO‐BP86 and ZORA‐SO‐PBE0 level, the occurrence of relativistic effects on the 1H, 13C, and 15N NMR chemical shifts of 1‐methylpyridinium halides [MP][X] and 1‐butyl‐3‐methylpyridinium trihalides [BMP][X3] ionic liquids (ILs) (X=Cl, Br, I) as a result of a non‐covalent interaction with the heavy anions. Our results indicate a sizeable deshielding effect in ion pairs when the anion is I− and I3−. A smaller, though nonzero, effect is observed also with bromine while chlorine based anions do not produce an appreciable relativistic shift. The chemical shift of the carbon atoms of the aromatic ring shows an inverse halogen dependence that has been rationalized based on the little C‐2s orbital contribution to the σ‐type interaction between the cation and anion. This is the first detailed account and systematic theoretical investigation of a relativistic heavy atom effect on the NMR chemical shifts of light atoms in the absence of covalent bonds. Our work paves the way and suggests the direction for an experimental investigation of such elusive signatures of ion pairing in ILs.

Evaluation of bismuth-based dispersion energy donors – synthesis, structure and theoretical study of 2-biphenylbismuth(iii) derivatives

“…It seems likely that the increasing downfield shift is mainly due to the so-called Inverse Halogen Dependence (IHD) which is caused by spin orbit coupling on the heavy halogen atom and its effect on the 1 H NMR shift. Similar effects for through space interactions and an IHD have been reported for iodo alkyl and aryl compounds by Kaupp et al and for ion pairs by Ariai et al 52,53 The 13 C{ 1 H} NMR spectra of the di-and monoarylbismuth(III) halides also exhibit a downfield shift for the C-6 carbon resonance signals, which follows the order Fig. S12, ESI †).…”

Relativistic Heavy-Neighbor-Atom Effects on NMR Shifts: Concepts and Trends Across the Periodic Table

“…The possibility of “through-space” propagation of SO-HALA effects in pyridinium halide ionic liquids has been investigated computationally . Using various computational models, δ SO ( 13 C) up to +3 ppm was predicted at the pyridinium carbon atoms when pyridinium was involved in an ion pair with I 3 – , but this effect vanished with the Cl 3 – analog.…”

“…The possibility of "through-space" propagation of SO-HALA effects in pyridinium halide ionic liquids has been investigated computationally. 211 Using various computational models, δ SO ( 13 C) up to +3 ppm was predicted at the pyridinium carbon atoms when pyridinium was involved in an ion pair with I 3 − , but this effect vanished with the Cl 3 − analog. The authors concluded that it may be challenging to find experimental evidence to support these computational predictions, owing to the short lifetime of the investigated ion pairs in a liquid environment.…”